U.S. Pat. Nos. 7,799,742, 7,345,010 and 6,339,048 as well as US patent applications US 2015/0376490 A1 and US 2013/0331303 A1 disclose polyamide additives for rheological control in oil-based drilling fluids, which are reaction products of di- or polyamines and di- or polycarboxylic acids. However, in particular in drilling fluids with highly non-polar continuous phases—such as drilling fluids containing synthetic olefin hydrocarbons (e.g. the commercial product Amodrill® 1000)—the use of such polyamides as rheological additives can lead to an only poor solvation or precipitation of the rheological additive due to the polarity of the amide bonds of the additive. This is in particular a disadvantage, because there is a trend towards the use of such highly non-polar drilling fluids because of ecological reasons (e.g., the use of more polar fluids containing aromatic hydrocarbons is no longer envisaged). Further, the tuning of the polarity of such polyamide additives is difficult due to the pricing and the limited availability of fatty acids used for the preparation of such polyamide additives.
U.S. Pat. No. 7,449,430 relates to water-based drilling fluids and discloses the use of non-functional styrene-acrylate or styrene-butadiene based copolymers as fluid loss reducers. U.S. Pat. No. 7,351,681 is directed at a well bore servicing fluid such as a drilling fluid comprising an ionic water-soluble, hydrophobically modified polymer.
U.S. Pat. No. 6,861,393 discloses non-functional polyalkyl methacrylate polymers as additives for reducing sag in oil-based drilling fluids. Non-functional, styrene-based (co)polymers for use as rheological agents in oil-based drilling fluids are disclosed in WO 2014/009408 A1. U.S. Pat. No. 5,629,270 discloses an oil-based drilling fluid, which inter alia contains a non-sulfonated polystyrene. U.S. Pat. No. 7,285,515 is directed at oil-based drilling fluids, which inter alia contain fluid loss polyacrylate additives. U.S. Pat. No. 3,506,574 discloses an oil-based drilling fluid, which contains a graft copolymer of at least one (meth)acrylate, wherein N-vinyl pyrrolidone is grafted on the backbone of said copolymer.
The use of (meth)acrylic copolymers, which may but do not necessarily have to contain carboxylic acid functionalities, in oil-based drilling fluids is disclosed in WO 00/78891 A1. The copolymers according to WO 00/78891 A1 are not used as rheological agents, but are used as fluid loss additives. U.S. Pat. No. 4,548,982 discloses water loss additives for drilling fluids, which are ammonia salts of copolymers of (meth)acrylic acid and at least one of (meth)acrylic acid esters and vinyl esters of saturated monocarboxylic acids having 1 to 3 carbon atoms.